PERIODICITY IN HUMAN BEINGS AND MICE 833 



dary endocrine and metabolic changes are prominent (Ferguson et ah, 



1957). 



Earlier in this symposium, Doctor Bullough clearly sketched the 

 links involved in the mediation of effects of light upon the sex cycle. 

 His thoughts come to mind, while we face the task of examining the 

 role of those structures that mediate and modulate effects exerted by 

 lighting schedules upon the phase of 24-hr rhythms. We start out, as 

 Doctor Bullough did, with synchronizing environmental stimuli, such 

 as light or sound, that are received by "transducers" such as the eye 

 or the ear, respectively. Potentials thus evoked are transmitted corti- 

 cipitally, by conductors to centers in the brain, but, for the moment, 

 we need not travel all the way to the striate area or to other special 

 sensory areas of the cerebral cortex. For the conduction of photic 

 effects, it is of interest that some fibers of the optic nerve go to the 

 hypothalamus and to the reticular formation. For the conduction of 

 visual, as well as of auditory, visceral, and somatic input, it also seems 

 pertinent that impulses evoked by these various peripheral afferent 

 stimuli can be conducted corticipitally through the central brain stem 

 (Allen, 1923; Bremer, 1935; French et al, 1952, 1953a,b; Ingram 

 etal, 1951;Magoun, 1952; O'Leary and Coben, 1958; Russell, 1957; 

 Starzl et al, 1951), as well as over classic (lateral) sensory pathways. 

 Moreover, potentials recorded medially show a lack of modality segre- 

 gation, interaction and attenuation of succeeding responses on multi- 

 ple stimulation, and diffuse cortical distribution, among other charac- 

 teristics which make them particularly attractive to the student of 

 24-hr periodicity. Is it at the site of these potentials, perhaps in the 

 reticular formation of the brain that, after the removal of biologic 

 "noise," the multitude of synchronizing impulses reaching nervous 

 centers is mixed, weighted, and combined? These are important func- 

 tions, since we are exposed, often at the same time, to a variety of 

 stimuli, yet ordinarily only one stimulus will be dominant, a few will 

 be modulating, while other sensory input remains silent with respect 

 to phase of rhythms. Such biologic noise removal, as well as mixing, 

 weighting, and combining, must go on continuously, since the domi- 

 nance of a given stimulus is never "dictatorial." To cite an example 

 from the Minnesota work, ordinarily the lighting regimen is the 

 dominant synchronizer of the eosinophil rhythm in mice (Halberg, 



